This invention relates in general to floating vessels that require a stable platform for working above a reference location in a body of water, and in particular the invention relates to vessels such as deep-sea drilling units which are adapted for station keeping above a reference location such as a wellhead on the ocean floor.
Various designs have been developed for ocean going, station keeping vessels in operations such as offshore oil and gas exploration and drilling. Among these designs is the conventional offshore drilling platform which is supported on legs imbedded in the sea bed. However, drilling platforms of this nature are restricted to relatively shallow depths such that their use is infeasible for water depths greater than about 300 feet. For greater depths a number of different designs have been developed in which a floating ship or surface vessel is sailed to the underwater drilling location and is maintained above the desired site while the drilling operation progresses. Drilling ships with conventional hull designs have been used for this purpose by controlling their propulsion units to continually maintain position over the wellhead against environmental forces such as wind, current and wave action. However, ships of this nature are roll sensitive so that they must maintain a heading into the direction of the waves. In most cases such a heading is not favorable to minimize the effects of wind and current, so that additional power must be expended to counter these latter forces while maintaining the drilling position. Semi-submersible drilling units have also been provided which incorporate a drilling platform supported above the water surface by columns mounted on one or more submerged hulls. In such a design the columns are of relatively small cross sectional area so that they are somewhat "invisible" to wave action, with the result that the vessel can maintain a heading which is optimum for current and/or wind conditions without regard to wave direction. Semi-submersible vessels of this nature, however, have been held in station keeping position by mooring lines anchored to the sea bed. Existing semi-submersible vessels of this type have no provision for continuously seeking an optimum heading as environmental forces change.